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If your son is *quite* slim, it's possible that the 40 lb weight limit on the Chase harness would be all you'd need to get him to an appropriate booster age. If the sale you are looking at is really good, it might be worth the risk that he'd need another seat later, and the Chase would buy him a little more harness height than the Britax. The tricky thing with the Chase, though, is that you pretty much *have* to outgrow it by weight if you want to be able to use it as a booster as well. If he outgrows it by height before he hits 40 lbs, then he isn't eligible to use it as a booster. How much does he weigh now and what is his seated shoulder height? Do you have a younger child that you could pass the Chase down to if you are able to get a Pioneer or other taller/higher weight capacity seat for him later?
When you say that he is ready to move out of the Britax, is that because he wants a more grown-up looking booster? or is he actually close to outgrowing the Britax?
If he hasn't outgrown his Britax convertible, you might be best off keeping him in it and seeing if he can stay in it until he's ready to learn how to use a dedicated booster. I don't know the rules in Canada for harness vs boostering, but at his age it would be considered safe to try a booster as long as it fits him properly, he can follow the booster rules, and he meets the requirements of the manufacturer.
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